Is there a way to convert 2.7x files to 2.4x?
While a lot of new data may not translate well into 2.49 as ideasman42 explains, there is an option if you only want to transfer mesh objects.

When you use 'File->Save As' there is a 'Use Legacy Mesh Format' option that will ensure no n-gon data is saved. This allows 2.49 to open a simple mesh with face data intact.